Update symbols listed in Version_script.in

There are 3 public symbols in the headers, which are missing in the
linker script and thus marked as private again. This causes issues for
e.g. gstreamer (https://bugzilla.gnome.org/show_bug.cgi?id=616194).

git-svn-id: https://svn.xiph.org/trunk/Tremor@17149 0101bb08-14d6-0310-b084-bc0e0c8e3800